1. Field of the Invention
This invention relates to a radiation image read-out method and apparatus. This invention particularly relates to a radiation image read-out method and apparatus, wherein a radiation image having been stored on a stimulable phosphor sheet is read out with a line sensor.
2. Description of the Related Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet, which comprises a substrate and a layer of the stimulable phosphor overlaid on the substrate. Stimulating rays, such as a laser beam, are deflected and caused to scan pixels in the radiation image, which has been stored on the stimulable phosphor sheet, one after another. The stimulating rays cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted successively from the pixels in the radiation image having been stored on the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal by photoelectric read-out means. The stimulable phosphor sheet, from which the image signal has been detected, is then exposed to erasing light, and radiation energy remaining thereon is thereby released.
Also, a novel radiation image recording and reproducing system aiming at enhancement of a detection quantum efficiency in the formation of the radiation image, i.e., a radiation absorptivity, alight emission efficiency, an emitted light pickup efficiency, and the like, has been proposed in, for example, Japanese patent Application No. 11(1999)-372978. With the proposed radiation image recording and reproducing system, the radiation absorbing functions and the energy storing functions of the conventional stimulable phosphor are separated from each other, and a phosphor having good radiation absorbing characteristics and a phosphor having good light emission response characteristics are utilized respectively for radiation absorption and radiation image storage. The phosphor having good radiation absorbing characteristics is caused to absorb the radiation and to emit light having wavelengths falling within a ultraviolet to visible region. Also, the phosphor having good light emission response characteristics is caused to absorb the light, which has been emitted by the phosphor having good radiation absorbing characteristics, and to store energy of the emitted light. The phosphor having good light emission response characteristics, on which the energy of the emitted light has been stored, is then exposed to light having wavelengths falling within a visible to infrared region, which light causes the phosphor having good light emission response characteristics to emit light in accordance with the stored energy. The light having thus been emitted by the phosphor having good light emission response characteristics is successively detected with photoelectric read-out means, and an image signal is thereby obtained.
The image signal, which has been obtained from the radiation image recording and reproducing systems described above, is then subjected to image processing, such as gradation processing and processing in the frequency domain, such that a visible radiation image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, can be obtained. The image signal having been obtained from the image processing is utilized for reproducing a visible image for diagnosis, or the like, on film or on a high resolution cathode ray tube (CRT) display device. In cases where the stimulable phosphor sheet, from which the image signal has been detected, is then exposed to the erasing light, and energy remaining on the stimulable phosphor sheet is thereby released, the erased stimulable phosphor sheet is capable of being used again for the recording of a radiation image.
Novel radiation image read-out apparatuses for use in the radiation image recording and reproducing systems described above have been proposed in, for example, Japanese Unexamined patent publication Nos. 60(1985)-111568, 60(1985)-236354, and 1(1989)-101540. In the proposed radiation image read-out apparatuses, from the point of view of keeping the emitted light detection time short, reducing the size of the apparatus, and keeping the cost low, a line light source for irradiating linear stimulating rays onto a stimulable phosphor sheet is utilized as a stimulating ray source, and a line sensor comprising a plurality of photoelectric conversion devices arrayed along the length direction of a linear area of the stimulable phosphor sheet, onto which linear area the stimulating rays are irradiated by the line light source, is utilized as photoelectric read-out means. (The length direction of the linear area of the stimulable phosphor sheet will hereinbelow be referred to as the main scanning direction.) Also, the proposed radiation image read-out apparatuses comprise scanning means for moving the stimulable phosphor sheet with respect to the line light source and the line sensor and in a direction, which is approximately normal to the length direction of the linear area of the stimulable phosphor sheet. (The direction, which is approximately normal to the length direction of the linear area of the stimulable phosphor sheet, will hereinbelow be referred to as the sub-scanning direction.)
FIG. 5 is an explanatory view showing spread of light emitted by a stimulable phosphor sheet. As illustrated in FIG. 5, in the cases of a transmission type of constitution, wherein a line light source and a line sensor are located on opposite surface sides of a stimulable phosphor sheet, a stimulable phosphor layer 50a of a stimulable phosphor sheet 50xe2x80x2, on which a radiation image has been stored, is exposed to stimulating rays L having been produced by a stimulating ray source and is caused to emit light M in proportion to the amount of stored energy carrying radiation image information. Also, the emitted light M passes through a substrate 50b capable of transmitting the emitted light M and is projected onto each of photoelectric conversion devices 21, 21, . . . acting as light receiving devices. In such cases, for example, after the stimulating rays L impinge upon the stimulable phosphor sheet 50xe2x80x2, the stimulating rays L are scattered within the stimulable phosphor sheet 50xe2x80x2. Also, the emitted light M, which has been produced by the stimulable phosphor sheet 50xe2x80x2 when the stimulable phosphor sheet 50xe2x80x2 is exposed to the stimulating rays L, is scattered within the stimulable phosphor sheet 50xe2x80x2 before the emitted light M is radiated out from the surface of the stimulable phosphor sheet 50xe2x80x2. Due to such reasons, a width dM of the emitted light M becomes larger than a width dL of the stimulating rays L, and the emitted light M spreads inevitably.
The same problems as in the cases of the transmission type of constitution also occur in the cases of a reflection type of constitution, wherein the line light source and the line sensor are located on an identical surface side of a stimulable phosphor sheet.
As illustrated in FIG. 5, in cases where the emitted light M is collected by a line sensor, wherein a width dP of each photoelectric conversion device 21, which width dP is taken in the direction normal to the main scanning direction, is smaller than the width dM of the emitted light M, which width dM is taken in the beam width direction of the emitted light M, the light collecting efficiency cannot be kept high due to leakage of the emitted light M, and therefore an image having good image quality cannot be obtained. Accordingly, how the spread of the emitted light M is ascertained and how the emitted light M is collected such that the amount of light, which does not impinge upon the photoelectric conversion device 21, may be reduced are the important problems for determining the image quality of the obtained image.
FIG. 6 is a graph showing an intensity distribution of light emitted by a stimulable phosphor sheet. As illustrated in FIG. 6, the intensity of the emitted light M is high at the center region of the beam of the emitted light M, which center region corresponds to the width dL of the stimulating rays L, and becomes low at positions spaced outwardly from the optical axis of the stimulating rays L. Since noise becomes relatively strong in weak light, it is important that the amount of the emitted light M collected is kept large, and the emitted light as close to the center region of the beam of the emitted light M as possible is collected by considering the balance with the cost and noise.
In U.S. Pat. No. 6,507,049, the applicant proposed a technique, wherein multiple rows of photoelectric conversion devices constituting a line sensor are arrayed such that the multiple rows stand side by side with respect to a direction normal to the main scanning direction, and a light receiving width of the line sensor, which light receiving width is taken in the direction normal to the main scanning direction, is thereby kept large. However, with the proposed technique, a charge transfer region, which is not capable of receiving the emitted light M, is located between light receiving regions of adjacent rows of the photoelectric conversion devices, which adjacent rows stand side by side with respect to the direction normal to the main scanning direction. Therefore, the width of the line sensor, which width is taken in the direction normal to the main scanning direction, is large, but the problems occur in that the efficiency, with which the emitted light at the center region of the beam of the emitted light close to the optical axis of the stimulating rays L is collected, cannot be kept high.
The primary object of the present invention is to provide a radiation image read-out method, wherein emitted light at a center region of a beam of the light emitted by a stimulable phosphor sheet, at which center region an intensity of the emitted light is high, is capable of being collected efficiently, a substantial light receiving width of a line sensor is capable of being kept large, and an image having good image quality is capable of being obtained.
Another object of the present invention is to provide an apparatus for carrying out the radiation image read-out method.
The present invention provides a radiation image read-out method, comprising the steps of:
i) linearly irradiating stimulating rays, which have been produced by a line light source, onto an area of one surface of a stimulable phosphor sheet, on which a radiation image has been stored, the stimulating rays causing the stimulable phosphor sheet to emit light in proportion to an amount of energy stored thereon during its exposure to radiation,
ii) receiving light, which is emitted from the linear area of the one surface of the stimulable phosphor sheet exposed to the linear stimulating rays or from a linear area of the other surface of the stimulable phosphor sheet corresponding to the linear area of the one surface of the stimulable phosphor sheet, with a line sensor, the received light being subjected to photoelectric conversion performed by the line sensor,
iii) moving the stimulable phosphor sheet with respect to the line light source and the line sensor and in a direction different from a length direction of the linear area of the stimulable phosphor sheet, and
iv) successively reading outputs of the line sensor in accordance with the movement,
wherein the line sensor comprises a pair of one-dimensional charge coupled device image sensors, and
light receiving regions of the one-dimensional charge coupled device image sensors are located close to each other so as to stand side by side with respect to a direction, which is normal to the length direction of the linear area of the stimulable phosphor sheet, and with a fine gap intervening between the light receiving regions.
The present invention also provides a radiation image read-out apparatus for carrying out the radiation image read-out method in accordance with the present invention. Specifically, the present invention also provides a radiation image read-out apparatus, comprising:
i) a line light source for linearly irradiating stimulating rays onto an area of one surface of a stimulable phosphor rays causing the stimulable phosphor sheet to emit light in proportion to an amount of energy stored thereon during its exposure to radiation,
ii) a line sensor for receiving light, which is emitted from the linear area of the one surface of the stimulable phosphor sheet exposed to the linear stimulating rays or from a linear area of the other surface of the stimulable phosphor sheet corresponding to the linear area of the one surface of the stimulable phosphor sheet, and performing photoelectric conversion of the received light,
iii) scanning means for moving the stimulable phosphor sheet with respect to the line light source and the line sensor and in a direction different from a length direction of the linear area of the stimulable phosphor sheet, and
iv) reading means for successively reading outputs of the line sensor in accordance with the movement,
wherein the line sensor comprises a pair of one-dimensional charge coupled device image sensors, and
light receiving regions of the one-dimensional charge coupled device image sensors are located close to each other so as to stand side by side with respect to a direction, which is normal to the length direction of the linear area of the stimulable phosphor sheet, and with a fine gap intervening between the light receiving regions.
In the radiation image read-out method and apparatus in accordance with the present invention, as the line light source, a fluorescent lamp, a cold cathode fluorescent lamp, a light emitting diode (LED) array, or the like, may be employed. The line light source may be a light source having a linear shape as in the cases of the fluorescent lamp. Alternatively, the line light source may be a light source operating such that the produced stimulating rays are formed into a line light beam. For example, the line light source may be a broad area laser, or the like. The stimulating rays radiated out from the line light source may be radiated out continuously. Alternatively, the stimulating rays radiated out from the line light source may be radiated out as pulsed stimulating rays, which are radiated out intermittently. From the point of view of reducing noise, the stimulating rays should preferably be pulsed stimulating rays having a high intensity.
As will be understood from the specification, it should be noted that the term xe2x80x9cmoving a stimulable phosphor sheet with respect to a line light source and a line sensorxe2x80x9d as used herein means movement of the stimulable phosphor sheet relative to the line light source and the line sensor, and embraces the cases wherein the stimulable phosphor sheet is moved while the line light source and the line sensor are kept stationary, the cases wherein the line light source and the line sensor are moved while the stimulable phosphor sheet is kept stationary, and the cases wherein both the stimulable phosphor sheet and the line light source and the line sensor are moved. In cases where the line light source and the line sensor are moved, they should be moved together with each other.
The direction along which the stimulable phosphor sheet is moved with respect to the line light source and the line sensor (i.e., the direction different from the length direction of the exposed linear area of the stimulable phosphor sheet) should preferably be the direction approximately normal to the length direction of the exposed linear area of the stimulable phosphor sheet (i.e., should preferably be the minor axis direction). However, the direction along which the stimulable phosphor sheet is moved with respect to the line light source and the line sensor is not limited to the minor axis direction. For example, the stimulable phosphor sheet may be moved with respect to the line light source and the line sensor along an oblique direction with respect to the direction approximately normal to the length direction of the line light source and the line sensor or along a zigzag movement direction, such that approximately the entire surface of the stimulable phosphor sheet may be uniformly exposed to the stimulating rays.
The line light source and the line sensor may be located on the same surface side of the stimulable phosphor sheet or on opposite surface sides of the stimulable phosphor sheet. In cases where the line light source and the line sensor are located on opposite surface sides of the stimulable phosphor sheet, the substrate of the stimulable phosphor sheet, or the like, should be formed from a material permeable to the emitted light, such that the emitted light may permeate to the surface side of the stimulable phosphor sheet opposite to the surface on the stimulating ray incidence side.
In the radiation image read-out method and apparatus in accordance with the present invention, the outputs of respective pixels of the light receiving regions of the one-dimensional charge coupled device image sensors (one-dimensional CCD image sensors) may be fed into an image processing unit, such that it is clear which outputs correspond to which site on the stimulable phosphor sheet. Also, in the image processing unit, operation processing may be performed on the outputs of the one-dimensional CCD image sensors, which outputs correspond to an identical site on the stimulable phosphor sheet, in order to obtain an output corresponding to the identical site on the stimulable phosphor sheet. In the radiation image read-out method and apparatus in accordance with the present invention, addition processing should preferably be performed (with operation processing means) on the outputs of pixels of the light receiving regions of the one-dimensional CCD image sensors, which pixels are located close to each other so as to stand side by side with respect to the direction normal to the length direction of the linear area of the stimulable phosphor sheet.
The addition processing may be simple addition processing, mean calculation processing, or weighted addition processing.
Also, in the radiation image read-out method and apparatus in accordance with the present invention, the stimulable phosphor sheet for storing the radiation image may be an ordinary stimulable phosphor sheet comprising a stimulable phosphor for absorbing radiation and storing energy from the radiation, i.e. the radiation image.
Further, the radiation image read-out method and apparatus in accordance with the present invention may be employed in the radiation image recording and reproducing system proposed in, for example, Japanese patent Application No. 11(1999)-372978. With the proposed radiation image recording and reproducing system, the radiation absorbing functions and the energy storing functions of the conventional stimulable phosphor are separated from each other, and a phosphor having good radiation absorbing characteristics and a phosphor having good light emission response characteristics are utilized respectively for radiation absorption and radiation image storage. The phosphor having good radiation absorbing characteristics (i.e., a phosphor for radiation absorption) is caused to absorb the radiation and to emit light having wavelengths falling within a ultraviolet to visible region. Also, the phosphor having good light emission response characteristics (i.e., a phosphor for energy storage) is caused to absorb the light, which has been emitted by the phosphor having good radiation absorbing characteristics, and to store energy of the emitted light. The phosphor having good light emission response characteristics, on which the energy of the emitted light has been stored, is then exposed to light having wavelengths falling within a visible to infrared region, which light causes the phosphor having good light emission response characteristics to emit light in accordance with the stored energy. The light having thus been emitted by the phosphor having good light emission response characteristics is successively detected with photoelectric read-out means, and an image signal is thereby obtained. With the proposed radiation image recording and reproducing system, the detection quantum efficiency in the formation of the radiation image, i.e., the radiation absorptivity, the light emission efficiency, the emitted light pickup efficiency, and the like, is capable of being enhanced as a whole. Therefore, in the radiation image read-out method and apparatus in accordance with the present invention, the stimulable phosphor sheet should preferably contain the phosphor for energy storage described above.
The phosphor for energy storage absorbs the light having wavelengths falling within the ultraviolet to visible region, which light has been emitted by the phosphor for radiation absorption, and stores the energy of the emitted light as the image information. The light having wavelengths falling within the ultraviolet to visible region is the light emitted by the phosphor for radiation absorption when the phosphor for radiation absorption absorbs the radiation. Therefore, the image information having been stored on the phosphor for energy storage is taken as the radiation image.
Furthermore, in the radiation image read-out method and apparatus in accordance with the present invention, positions of pixels of one of the one-dimensional CCD image sensors and positions of pixels of the other one-dimensional CCD image sensor should preferably be shifted from each other in the length direction of the linear area of the stimulable phosphor sheet.
Also, in the radiation image read-out method and apparatus in accordance with the present invention, a width of the fine gap between the light receiving regions of the one-dimensional CCD image sensors, which width is taken in the direction normal to the length direction of the linear area of the stimulable phosphor sheet, should preferably be smaller than the width of each of the two light receiving regions, which width is taken in the direction normal to the length direction of the linear area of the stimulable phosphor sheet.
Further, in the radiation image read-out method and apparatus in accordance with the present invention, such that the degree of light collection by the line sensor may be enhanced, a sum of widths of the two light receiving regions and a width of the fine gap, which widths are taken in the direction normal to the length direction of the linear area of the stimulable phosphor sheet, should preferably be larger than a beam diameter of the stimulating rays, which beam diameter is taken in the direction normal to the length direction of the linear area of the stimulable phosphor sheet.
Furthermore, such that the degree of light collection, with which the light emitted from respective sites on the stimulable phosphor sheet is collected by the line sensor, may be enhanced, the radiation image read-out method and apparatus in accordance with the present invention should preferably be modified such that a light guiding optical system for guiding the light, which is emitted by the stimulable phosphor sheet, to a light receiving surface of the line sensor is located between the stimulable phosphor sheet and the line sensor, and the light guiding optical system is either one of an equi-magnification optical system and a magnifying optical system.
Also, in the radiation image read-out method and apparatus in accordance with the present invention, a stimulating ray cut-off filter (a sharp cut-off filter or a band-pass filter) for transmitting only the light emitted by the stimulable phosphor sheet and filtering out the stimulating rays should preferably be located between the stimulable phosphor sheet and the line sensor. In this manner, the stimulating rays should preferably be prevented from impinging upon the line sensor.
With the radiation image read-out method and apparatus in accordance with the present invention, the line sensor for performing photoelectric conversion comprises the pair of the one-dimensional CCD image sensors, and the light receiving regions of the one-dimensional CCD image sensors are located close to each other so as to stand side by side with respect to the direction, which is normal to the length direction of the linear stimulating rays irradiated from the line light source, and with the fine gap intervening between the light receiving regions. Therefore, no charge transfer region intervenes between the light receiving regions of the line sensor as a whole. Accordingly, the emitted light at the center region of the beam of the emitted light close to the optical axis of the stimulating rays, at which center region the intensity of the emitted light is high, is capable of being collected efficiently. Also, the substantial light receiving width of the line sensor is capable of being kept large, and the efficiency, with which the emitted light is collected, is capable of being kept high. As a result, an image having good image quality is capable of being obtained.
Also, with the radiation image read-out method and apparatus in accordance with the present invention, the emitted light at the center region of the beam of the emitted light close to the optical axis of the stimulating rays, at which center region the intensity of the emitted light is high, is primarily collected. Therefore, even if the weak emitted light at regions comparatively remote from the optical axis of the stimulating rays is not collected, the efficiency, with which the emitted light is collected, is capable of being kept high. Accordingly, noise is capable of being suppressed, and the size of the radiation image read-out apparatus is capable of being kept small.
With the radiation image read-out method and apparatus in accordance with the present invention, wherein the stimulable phosphor sheet contains the phosphor for energy storage described above, the image quality of the obtained image is capable of being enhanced even further.
With the radiation image read-out method and apparatus in accordance with the present invention, wherein the positions of the pixels of one of the one-dimensional CCD image sensors and the positions of the pixels of the other one-dimensional CCD image sensor are shifted from each other in the length direction of the linear area of the stimulable phosphor sheet, variation in quality among the pixels of the line sensor is capable of being compensated for, and therefore uniformity of the line sensor as a whole is capable of being kept high.
With the radiation image read-out method and apparatus in accordance with the present invention, wherein the width of the fine gap between the light receiving regions of the one-dimensional CCD image sensors, which width is taken in the direction normal to the length direction of the linear area of the stimulable phosphor sheet, is smaller than the width of each of the two light receiving regions, which width is taken in the direction normal to the length direction of the linear area of the stimulable phosphor sheet, photoelectric conversion with a high aperture ratio is capable of being performed.
With the radiation image read-out method and apparatus in accordance with the present invention, wherein the sum of the widths of the two light receiving regions and the width of the fine gap, which widths are taken in the direction normal to the length direction of the linear area of the stimulable phosphor sheet, is larger than the beam diameter of the stimulating rays, which beam diameter is taken in the direction normal to the length direction of the linear area of the stimulable phosphor sheet, at least the emitted light at the region of the beam of the emitted light, at which region the intensity of the emitted light is high, is capable of being collected. Therefore, the light collecting efficiency is capable of being enhanced even further.